The thermodynamic and other physical properties of materials that are confined to essentially nano-scale dimensions, such as organic (polymeric and biological) and inorganic ultra-thin (essentially two-dimensional or surface) films and nano-particles, differ significantly from those of bulk (essentially three-dimensional) materials. For example, organic ultra-thin films and nano-particles typically have heat capacities that are relatively smaller and demonstrate thermal transitions and changes over relatively broader ranges, with relatively shallow thermal transitions and changes. This makes the measurement and characterization of heat capacities, thermal transitions (such as melting points, glass transition temperatures, and the like), and thermal changes associated with the formation of new products (such as heats of reaction in single and multi-layer samples and the like) more difficult. In addition, the heats involved are typically on the order of about 1 nJ or less. Thus, conventional devices and methods used to measure and characterize the thermodynamic and other physical properties of bulk materials, such as conventional differential scanning calorimeters and the like, are inadequate for the measurement and characterization of the thermodynamic and other physical properties of ultra-thin films and nano-particles due to their lack of sensitivity, and because such devices and methods are not used at near-adiabatic conditions. In general, these devices and methods incorporate relatively large thermal mass addenda and time constants.
A number of nano-calorimeter devices have been developed and fabricated to measure and characterize the thermodynamic and other physical properties of ultra-thin films and nano-particles. However, most of these nano-calorimeter devices suffer from undesirable thermal leaks. Most of the conventional nano-calorimeter devices incorporate a plurality of micro-mechanical sensors, polymeric membranes, or thin film silicon nitride (SiNx) membranes on which one or more samples are deposited. Experiments are performed under vacuum conditions in order to minimize thermal leakage by convection and ultra-fast heat pulses are used in order to make thermal leakage by conduction negligible in comparison with the heating rates.
For example, Efremov et al. (“Discrete Periodic Melting Point Observations for Nanostructure Ensembles,” Physical Review Letters, Vol. 85, No. 17, pp. 3560–3563 (Oct. 22, 2000)) disclose a nano-calorimeter device that includes a thin film (30 nm) silicon nitride (SiNx) membrane that is several millimeters wide. Two metallic strips consisting of Ni, Au, or Pt, each with a thickness of 50 nm and a width of 400 μm, are deposited on one side of the silicon nitride membrane and serve as both micro-heaters and resistive thermometers. By using a thin silicon nitride membrane as the support system, the sensor has relatively low thermal mass addenda. The variation of the resistance of the micro-heaters with temperature is calibrated prior to use. Relatively fast heating rates (up to 106 K/s) are used, minimizing conductive and radiative heat losses. Thus, the nano-calorimeter device, including the metallic strips, a sample ultra-thin film deposited directly on the surface of the silicon nitride membrane and adjacent to one of the metallic strips, and a portion of the silicon nitride membrane itself, is operated at near-adiabatic conditions. Calorimetric measurements are performed in a differential scanning mode, with one of the metallic strips serving as a reference sensor. Calorimetric measurements proceed by applying a current pulse to both of the metallic strips, sample and reference, simultaneously. The voltage and current across the micro-heaters are measured and used to calculate power, temperature, and heat capacity in, for example, a study of melting points.
Kwan et al. (“Nanoscale Calorimetry of Isolated Polyethylene Single Crystals,” Journal of Polymer Science: Part B: Polymer Physics, Vol. 39, pp. 1237–1245 (2001)) disclose a nano-calorimeter device that includes a thin film (30 nm) amorphous silicon nitride (a-Si3N4-x) membrane supported by a silicon frame. A thin (50 nm) patterned Pt strip (500 μm×5 mm, ˜70 Ω) is deposited on one side of the silicon nitride membrane and used as both a micro-heater and a resistive thermometer. The material of interest is deposited on the silicon nitride-side of the silicon nitride membrane, adjacent to the micro-heater/thermometer. Differential scanning calorimetry is performed after calibration using two identical sensors in a common setup: a sample sensor (with the material of interest) and a reference sensor (without the material of interest). The calorimetric measurement of, for example, melting points is initiated with the application of a synchronized direct-current (DC) electrical pulse (9–25 mA, 2–10 ms) to each micro-heater. High heating rates (2×104−2×105 degrees C./s) under high-vacuum conditions (˜10−6 Torr) allow the measurements to approach adiabatic conditions.
While existing nano-calorimeter devices allow for higher sensitivities and shorter response times than conventional differential scanning calorimeters in measuring phase transition temperatures and heat capacity changes, these nano-calorimeter devices have not been designed and optimized to obtain highly-accurate quantitative calorimetric measurements. For example, in designs proposed and used by Allen et al. (“The Design and Operation of a MEMS Differential Scanning Nanocalorimeter for High-Speed Heat Capacity Measurements of Ultrathin Films,” Journal of Microelectromechanical Systems, Vol. 12, No. 3, pp. 355–364, (June, 2003) and “Thin-Film Differential Scanning Calorimetry: A New Probe for Assignment of the Glass Transition of Ultrathin Polymer Films,” Macromolecules, Vol. 35, No. 5, pp. 1481–1483 (Feb. 26, 2002)), the sample and reference cells are either not physically connected or exist in close proximity to one another with no heat sink provided between them. Thus, what is needed is a nano-calorimeter device that allows for differential scanning measurements wherein, in a symmetrical configuration, inherent measurement errors due to thermal leakage are equal in both cells and easily counterbalanced. What are also needed are improved cell and micro-heater designs.
In fact, in each of the references described above, multiple approximations are made regarding the device and the sample in order to extract the heat capacity, introducing errors on the order of magnitude of the measurables. Examples of such approximations include: neglecting convective, conductive, and radiative thermal leakage under pulsing conditions; ignoring cross-talk between the micro-heaters; ignoring thermal lag for certain samples; assuming that interfacial stress effects are insignificant; etc. These approximations considerably simplify the design of the device at the expense of accuracy, sensitivity, resolution, and measurement repeatability. The number of approximations required may be considerably reduced by optimizing the design of the cells, sample, micro-heaters, and thermal shields used.
Thus, in general, what is still needed is an improved nano-calorimeter device that is simple, effective, and further minimizes thermal leakage due to convection, conduction, and radiation, allowing the nano-calorimeter device to operate at near-adiabatic conditions. The nano-calorimeter device should have increased sensitivity and decreased thermal mass. The nano-calorimeter device should also incorporate and utilize power compensation, eliminating the drift that is present in existing nano-calorimeter designs and allowing a more flexible material system to be used.